Gas lasing devices, such as excimer or exciplex lasers and amplifiers, use a gas or a gas mixture as a gain medium to amplify light and/or provide laser output. Such gas lasing devices may include a sealed pressure chamber that includes two main discharge electrodes, a gas circulation system, gas reservoir, at least one feed valve, and two windows or resonator optics. The reservoir and main discharge chamber contain one or more high purity, ionizable gases, which often must be periodically evacuated to a minimum pressure and refilled under typical operating conditions.
In a gas lasing device, the gases should each be at the correct partial pressure such that laser efficiency is maximized and output beam quality is consistent. For many gas laser types, the gas partial pressures are depleted during laser usage. In exciplex lasers using a highly reactive halogen gas, for example, the halogen gas is continuously depleted from the discharge chamber by binding with components (e.g., water or contaminants) within the discharge chamber or reservoir. Depletion of the gaseous gain medium presents a problem with performance, particularly in those lasing devices that utilize highly reactive halogen gases. During the course of a single laser gas fill lifetime, the output energy of the laser will decrease as a function of shot count and time due to, at least in part, the depletion of the reactive gas. Although the gases may be removed and refilled, the cost of refilling and/or the downtime required may significantly increase the cost of owning and operating gas lasing devices used in many industrial manufacturing applications. To prolong the life of a gas fill (or fill lifetime) in a gas laser or amplifier, the gases may be replenished as gases are depleted without having to empty and refill.
A gas laser may have a control system that monitors the output pulse energy of the laser and controls the high voltage supplied to the discharge electrodes such that the output remains constant. Alteration of the relative concentration of any one of the laser gases or a change in the total gas pressure with respect to the optimum will cause a reduction in output energy requiring an increase in high voltage to maintain laser pulse energy. An increase in the voltage may accelerate depletion of the gases, due to the generation of dust contaminants resulting from increased wear on the electrodes and a greater input of energy, most of which is dissipated as heat. Higher input voltage also modifies the optical properties of the laser beam, which may cause the end user to have less usable energy on target.
One method to extend the lifetime of the gaseous laser gain medium is to periodically add one or more of the laser gases throughout the laser gas fill lifetime at a predetermined upper limit increase in input voltage or some other parameter indicative of the depletion of one or more of the gases. Such discrete gas injection may extend the life of the laser but relies on monitoring a parameter that may have already degraded for other reasons. Such discrete gas injection techniques may also result in dilution of other constituent gases when only the most reactive component of a mixture of gases is depleted and replenished. Another drawback to existing discrete gas injection methods is the tendency of the laser discharge chamber gases to back-mix through the gas input system as pressures equilibrate. Back mixing of laser discharge gases will cause contamination of the gas source lines and will change the ratios of the gas mixtures in the gas input system, thereby reducing laser gas lifetime.
A further drawback of the discrete gas injection methods is the inherent variability of the laser operating conditions as a result of the sudden introduction of the gases. For example, input voltage increases gradually as output energy decreases due to depletion and contamination. The sudden introduction of additional gases causes the output energy to increase (and voltage to decrease) relatively abruptly, with a correlating change in beam character, all of which can have an influence on process repeatability and reliability.
Another method of replenishing amounts of laser gases includes constant, low levels of flushing of one or more laser gases throughout the laser run. Such methods of continuous gas replenishment have the potential to provide more stable performance of the laser. These methods have been employed in applications either with an algorithm used to calibrate response to a variety of parameters or with a filtration system designed to remove impurities and/ or one or more constituent gases prior to replenishing the gases in higher concentrations. Existing continuous gas replenishment methods, however, have required complex and expensive components to provide the improved stability. These complex components are also more likely to fail and require costly service by specialized factory technicians. Thus, the high cost and service demands often negate any advantage over the discrete injection methods.
Continuous gas replenishment is preferable to discrete gas injection because the laser is not subject to repeated cycles of gas depletion and recovery that can change beam parameters and shorten the gas lifetime. Another technique uses discrete injection amounts with the goal of decreasing the volume and interval such that the response to the injections nears that of a continuous injection but without using overly complicated and expensive continuous gas replenishment systems. This controlled discrete injection method, however, may result in back-mixing of the laser gas, contamination of the source gas lines, and a continuously changing effective rate of injection of reactive gas. This controlled discrete injection method may also lead to millions of cycles of wear on the injector valves, further increasing the possibility for component failure and system down time. As such, methods for monitoring changes in chamber operation as a means to control discrete gas injections have become increasingly complex and have produced no significant benefits to operational stability and cost reduction.
Thus, existing gas replenishment techniques rely on expensive equipment or operator judgment, may not be reliable or repeatable in an industrial environment, and can be prone to back-flow contamination of the gas supply manifold system.